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When reading a FrameSet from a FitsChan the CTYPEi values in the FitsChan are used to set the
Symbol attributes of the corresponding WCS Frame. The Label attributes of the WCS Frame are set from
the CNAMEi keywords, if present in the header. Otherwise they are set
from the CTYPEi comments strings in the header, so long as each
axis has a unique non-blank comment. Otherwise, the Label attributes are
set to the CTYPEi values. The above procedure is over-ridden if
the axis types conform to the conventions described in paper II or III,
as described below.
When writing a FrameSet to a FitsChan, each CTYPEi value is set to
the value of the Symbol attribute of the corresponding axis in the Frame
being written. If a value has been set explicitly for the axis Label
attribute, it is used as the axis comment (except that any existing
comments in the FitsChan take precedence if the keyword value has not
changed). The above procedure is over-ridden if the Frame is a
SkyFrame or a SpecFrame, in which
case the CTYPEi value is derived from the System
attribute of the Frame and the nature of the pixel to WCS Mapping
according to the conventions of papers II and III, as described below.
Next: Choice of Reference Point
Up: Paper I - General Linear Coordinates
Previous: Requirements for a Successful Write Operation
AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 210
R.F. Warren-Smith & D.S. Berry
24th May 2011
E-mail:ussc@star.rl.ac.uk
Copyright (C) 2009 Science \& Technology Facilities Council